To provide a conversion method of an image data for making a printing forme which is excellent in tone reproduction especially for printed matter of building material use and difficult to generate a trouble in printing such as clogging and the like.

A distribution range of a pixel value v in each color channel is examined independently each color channel to obtain the maximum value vmax of the distribution range. By making the printing forme based on image data to which conversion process of a pixel value wherein the pixel value v is, each pixel, made a value normalized by the maximum value vmax in the distribution range is applied, printing stability is raised to obtain the printing forme of an excellent decorative property.